Instructors that talk about shoulder turn to describe the amount of body turn during the backswing are hurting their students. What this means is if you start your backswing by pulling the club away with your hands, you may be hurting your swing right at the start of the backswing.
To explain this, I will ask you to try a little experiment.
Reach both arms in front of your body and take your right hand and grab your left wrist. Now, without turning your chest, pull your left arm across your chest with your right hand until your left arm is pointing to your right. Notice how far your left shoulder moved. If your shoulders do this during your backswing and you think you are turning, this is the root of your backswing maladies.
For the club face to come back to the ball square and centered, the geometry and relationship of the arms and upper body cannot change during the swing. Doing this experiment, you see that what might look like a backswing to many golfers, but is actually a major change in the arms/body geometry which can result in a whole slew of wild shots depending on what else the golfer tries to compensate.
Now try a new experiment.
Cross your arms across your chest and turn your upper body until your chest faces away from the target (make a backswing). You’ll notice that your arms and chest moved together. This is the same movement you should be making during your backswing.
What’s the difference between the two turns?
During the first turn, the arms pulled parts of the upper body until the arms looked as if they were in place. The stomach and hips didn’t turn. During the second turn, you had to turn your mid-section to make the turn, also turning the hips as needed. Those are the very same muscle you should always use to make your normal backswing turn.